On May 1st, 1999, a study was started regarding the spread of hepatitis B in the Netherlands (the so-called BRON study). For one year, data will be collected on new cases of hepatitis B with respect to transmission route, source of infection and risk factors. In addition, by use of mailed questionnaires, information on risk factors will be collected within a large group of control subjects throughout the country. The BRON study is being carried out by the National Institute for Public Health and the Environment (RIVM) in co-operation with the municipal health services and medical laboratories. The results of this study should underpin future governmental policy decisions regarding universal or selective vaccination against hepatitis B.